1. Field of the Invention
The present invention relates to determining network nodes that represent multiple subnetworks by aggregating their network addresses for routing of data packets across a network, and in particular to techniques for selecting an optimal set of one or more routers to aggregate network addresses of subnetworks within the network.
2. Description of the Related Art
Networks of general purpose computer systems connected by external communication links are well known and widely used in commerce. The networks often include one or more network devices that facilitate the passage of information between the computer systems. A network node is a network device or computer system connected by the communication links. An end node is a node that is configured to originate or terminate communications over the network. An intermediate network node facilitates the passage of data between end nodes.
Communications between nodes are typically effected by exchanging discrete packets of data. Information is exchanged within data packets according to one or more of many well known, new or still developing protocols. In this context, a protocol consists of a set of rules defining how the nodes interact with each other based on information sent over the communication links. Each packet typically comprises 1] header information associated with a particular protocol, and 2] payload information that follows the header information and contains information that may be processed independently of that particular protocol. In some protocols, the packet includes 3] trailer information following the payload and indicating the end of the payload information. The header includes information such as the source of the packet, its destination, the length of the payload, and other properties used by the protocol. Often, the data in the payload for the particular protocol includes a header and payload for a different protocol associated with a different layer of detail for information exchange. The header for a particular protocol typically indicates a type for the next protocol contained in its payload. The higher layer protocol is said to be encapsulated in the lower layer protocol.
The headers included in a packet traversing multiple heterogeneous networks, such as the Internet, typically include a physical (layer 1) header, a data-link (layer 2) header, an internetwork (layer 3) header and a transport (layer 4) header, as defined by the Open Systems Interconnection (OSI) Reference Model. The OSI Reference Model is generally described in more detail in Section 1.1 of the reference book entitled Interconnections Second Edition, by Radia Perlman, published September 1999, which is hereby incorporated by reference as though fully set forth herein.
The internetwork header provides information defining the source and destination address within the network. Notably, the path may span multiple physical links. The internetwork header may be formatted according to the Internet Protocol (IP), which specifies IP addresses of both a source and destination node at the end points of the logical path. Thus, the packet may “hop” from node to node along its logical path until it reaches the end node assigned to the destination IP address stored in the packet's internetwork header.
Some network layer addresses, including IP addresses are hierarchical and can be aggregated. Hierarchical addresses are organized into numerous groups and subgroups and subgroups of subgroups, etc. Each layer of subgroups successively narrow the address space until at, the finest level of granularity of the address space, a single element of the network is indicated (e.g., a network interface card on a network node). A group address aggregates the addresses in the subgroups of that group.
Routers and switches are network devices that determine which communication link or links to employ to support the progress of data packets through the network. A network node that determines which links to employ based on information in the internetwork header (layer 3) is called a router. Some protocols pass protocol-related information among two or more network nodes in special control packets that are communicated separately and which include a payload of information used by the protocol itself rather than a payload of data to be communicated for another application. These control packets and the processes at network nodes that utilize the control packets are said to be in another dimension, a “control plane,” distinct from the “data plane” dimension that includes the data packets with payloads for other applications at the end nodes.
A link-state protocol is an example of a routing protocol, which only exchanges control plane messages used for routing data packets sent in a different routed protocol (e.g., IP). In a link-state protocol, two routers establish an adjacency relationship between them by first verifying direct two-way communication between them over the same network segment and then synchronizing their link-state databases. Link-state data describe all links to a router and describes the network addresses reachable on each of those links, as well as other properties of the link. Once the adjacency relationship is established, the two routers are called peers. During a reliable flooding stage of a link-state protocol, each router is required to ensure that each of its peers has received the link-state data describing the router's own links.
As a consequence, resources on each router and many other network nodes are consumed for each set of addresses associated with each link. The resources consumed by the router include: memory to store the addresses associated with each link; processor time to compute a route based, at least in part, on the addresses reachable over each link; and both processor time and link bandwidth for sending, receiving and processing routing information involving the link. As the size of a network increases, the consumption of network resources also increases, interfering with the capacity of the protocol to scale up to a network with a large numbers of nodes. The capacity for a network protocol to scale up to a network with a large number of nodes is called the scalability of the protocol.
To reduce the consumption of network resources and improve scalability, some routing protocols divide a large network up into smaller subnetworks. For example, the Open System Interconnection (OSI) protocol suite and the Open Shortest Path First (OSPF) routing protocol divide a network into domains and areas. A domain is a portion of a network under the network administration of a single authority, such as an enterprise or Internet service provider (ISP). A domain is divided into areas. Each area is a group of contiguous subnetworks and attached end nodes specified by a network administrator. In OSI, routers communicate with each other using an intermediate system to intermediate system (IS-IS) protocol. According to IS-IS, routing within an area (level 1 routing) uses link-state data that distinguishes each link on each router in the area. Routing between areas (level 2 routing) goes through a level 2 router that aggregates the addresses reachable through that level 2 router. By aggregating routing information for addresses reachable over many links of a level 2 router, the amount of network resources consumed to maintain link-state data and make routing decisions can be reduced and network scalability can be enhanced.
The division of routers into areas is a manual process performed by human network administrators. As a result, the division is subjective based on the administrator's perceptions and is not guaranteed to be optimal in any objective sense. While this has been suitable for many past network domains, as networks become larger, approaching hundreds of thousands of nodes, the subjective approach to dividing areas becomes more tedious and error-prone. Furthermore, as networks become larger, sub-optimal divisions can lead to significant wasted resources and increased costs to service the same customer base for a network domain. In some circumstances, sub-optimal divisions can lead to instability and lack of resiliency in a network domain.
Based on the foregoing, there is a clear need for an objective approach to determining the division of a network domain into areas and the selection of routers to serve as routing information aggregation nodes. In general, there is a need for techniques to analyze a deployed or planned network and determine whether the configuration is optimal for the aggregation of address reachability and network topology information.